Various hydraulic control systems are commonly utilized in conventional downhole deployments. For example, one common hydraulic system makes use of the absolute value of a differential fluid pressure between drilling fluid internal to the drill string (or BHA) and drilling fluid in the borehole annulus to perform a tool function (e.g., reset a switch). Differential fluid pressure has also been utilized to actuate one or more blades in an adjustable stabilizer (U.S. Pat. No. 5,318,138). While such applications are commercially serviceable, the use of a differential pressure can be problematic. The pressure differential is known to be a function of various drilling factors, for example, including drilling fluid flow rate, velocity, and viscosity, size of the drill bit nozzles, the longitudinal distance of the hydraulic system from the drill bit, and the borehole diameter. Thus, the differential pressure can (and often does) vary widely within a drilling operation and from one drilling operation to the next. Such pressure variations are known to cause tool reliability issues. Furthermore, the above described hydraulic systems often require that the flow of drilling fluid in the drill string must be essentially stopped and restarted to perform the function.
More complex hydraulic control systems are also commonly utilized, for example, in rotary steering tools to control the radial position of and/or the lateral force applied to each of a plurality of steering blades. Such systems commonly include a hydraulic pumping mechanism (e.g., a cam driven piston pump) and numerous electronically controllable (e.g., solenoid) and pressure relief valves to maintain a constant (or a controllable) hydraulic fluid pressure. While such systems have been reliably used in downhole tools, they tend to be expensive to build and maintain due to their complexity. Therefore, they tend not to be suitable for certain downhole applications
There is a need in the art for a relatively inexpensive hydraulic control system for maintaining constant or near-constant hydraulic pressure. Such a system advantageously does not require a pumping mechanism or electronic controllable valves (e.g., solenoid valves) or other controllable components.